I've recently got an ebook reader - I decided to take a Kobo for a number of ressons:

pretty cheap

easily hackable (and it seems that they understand Open Source - Most of the software for their devices is on github - https://github.com/kobolabs )

no hard vendor lock-in (Well, they do have their kobo-store, but you are not forced to use it.)

First a Warning:

Hacking the firmware of embedded devices always includes the risk to break the device. And it may void Warranty.

If you can't live with that risk don't do it!

And always backup all your data first! If you screw something up you will need to do a factory reset - and this will wipe all data.

Why rsync? For backups - my tool of choice is dirvish. Of course I could connect the Kobo via USB, mount it and do the backup from there, but I prefer the direct way. And I can do a full system backup, not only the data.

Step 1) Getting a root Shell

The Kobo runs on Linux with busybox, so it has a telnet server already installed but not activated. So we don't need additional software for this step, we just need to change a few config files. To do this the update system can be misused - it's pretty simple: Connect via USB and put a KoboRoot.tgz into the .kobo folder. This file will then be extracted to / - without any kind signature checks or similar. Easy to use, but also easy to break stuff.

Some tutorials I found recommend to do a factory reset first, I did not and it just worked fine.

The hardest part is to find the download Url of the firmware package. You could try to run an update on your Kobo and sniff the network or use the (Win only) setup software and have a look what it downloads. Or just trust other people who have already collected the urls - like in this thread @ mobileread.com.

Unzip that and you will find KoboRoot.tgz besides other stuff we don't need now.

Tar this into a KoboRoot.tgz and put the file into .kobo on your device. After disconnecting USB it will install it and reboot. Turn on Wifi in the settings and start the Browser (it enables Wifi only on demand, starting the browser makes sure it really does) and you should be able to connect via telnet as root without password. The kobo is wide open now, do this only in a secure network.

Step 2) install the dropbear ssh server

So far I'm too lazy to set up a build environment for the Kobo, so I'm using binaries from the debian armhf port. After reading tjms comment I started with Maemo binaries. While they do work for dropbear they fail with rsync, more about that later.

Step 3) rsync

This took me a while since I started with Maemo packages - rsync just did exit 1 without any further notice, strace and LD_DEBUG did not really help, finally derRichard pointed me to the right direction: The Kobo uses EABI5, Maemo EABI4, so everything that needs additional shared libraries will fail.